Lean burn combustors, such as those used on modern aircraft engines, are susceptible to combustion dynamics. Under certain engine operating conditions, significant transient pressure waves (“pings”) can be present, particularly in an annular combustor. These pressure waves, if of sufficient magnitude, can cause high cycle fatigue of combustor components, long before the hardware would need to be replaced under normal operations.
One known approach to combustor dynamic issues can involve careful mapping of problem regimes using test engines with multiple combustor instrumentation pressure sensors. Aircraft fuel schedules developed from this process and subsequently programmed into engine control were expected to avoid all problem areas. Despite this mapping, subtle changes to an engine still can adversely affect combustion dynamics behavior. These changes can be due to parameters including manufacturing variations, engine deterioration, fuel composition, or unexpected flight conditions.
Other known approaches for monitoring combustor dynamics have been taken in marine and industrial turbine engines. For example, dedicated pressure sensors have been used to optimize fuel consumption and minimize emissions and occurrences of combustor acoustic resonance. Hardware resonators are sometimes added to mitigate specific modes in commonly used power regions for marine and industrial turbine engines. Such pressure sensors, hardware resonators and other related components introduce additional size and weight that may not be tolerable for an aviation engine. Combustor geometry is also significantly different in marine and industrial engines, so mode shapes and interactions are not directly applicable to an aviation application.
Specific aspects of combustor dynamics modeling in marine and industrial applications are often designed with different operational targets than for aircraft applications. For instance, marine and industrial turbine engines are typically run at a small number of operating points under steady state conditions for long periods of time. The sensors and monitoring logic are therefore set up for long-term averaging and slow response times, which may not always be ideal for aircraft applications. In some examples, sensors employ control logic that has a response time in a range from about a few seconds to as long as a minute or more for fuel control changes to mitigate resonance. Monitoring can use straightforward filtering to calculate a peak resonance value in one or two broadband areas, which is then fed back into the control logic. This approach does not attempt to pinpoint a specific frequency of the resonant mode(s) being excited.
Accordingly, features for monitoring combustion dynamics in aircraft engines are desired. Specifically, features for observing engine dynamics during transient engine operation in real time in order to identify specific resonant frequencies are desired.